What happens when the Tiny Servant and True Polymorph spells effects a single target but end at different times?

Question

I was inspired to ask this question from a post about using a tankard to fight people.

Here's the scenario: A tiny object, in this case a Tankard, has been turned into a tiny construct thanks to the Tiny Servant spell, and will remain as such for 8 hours (the spells duration). It is subsequently targeted by a True Polymorph turning it into a different creature, lets say a bat. True Polymorph states that if the caster maintains concentration for the full duration (1 hour), the effect becomes permanent. If such a circumstance occurred

What happens to the bat when Tiny Servant ends?

If it remains a bat, and is subsequently brought to 0 HP (not difficult, considering it only has 1 HP) does it return to being a tankard then, or is it just a dead bat?

Answer 1

The creature will remain a bat, even after it dies...

... ASSUMING you can do it in the first place. While a bat does have a CR of 0, we don't know what the CR of a tiny servant is. According to this Twitter question and answer, they don't have ANY challenge rating, but this is not official. I have not found anything about it in the Sage Advice Compendium, either.

But, assuming you can,

True Polymorph (emphasis mine):

You transform the creature into a different creature, the creature into an object, or the object into a creature

Just from this statement alone, there's nothing preventing you from transforming the tiny servant (a creature) into a bat permanently. Even after the 8 hours, when the tankard would normally go back to being an object, the spell says that it works on turning objects into creatures, so it's still not contradictory.

And according to this text:

The transformation lasts for the duration, or until the target drops to 0 hit points or dies. If you concentrate on this spell for the full duration, the transformation lasts until it is dispelled.

Since death is not a form of dispelling, it will remain a bat unless you dispell it first. (The corpse of the bat may or may not be able to be dispelled)

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Situations where this may not work

These are situations with true polymorph that would cause the target to no longer be valid, and we don't necessarily know what would happen. (per @Medix2 in the comments, This is discussed a bit here: "Is a spell suppressed or removed when the target temporarily becomes invalid?")

1. Let's assume that instead of a bat, you change the tankard into a ball. true polymorph doesn't let you transform an object into another object, causing the target to no longer become valid. The ball may stay a ball, since it was valid before the tiny servant spell wore off, but it may revert.
2. To keep it on track of morphing into 'B' words, let's morph it into a baboon instead. We're continuing the assumption that we're allowed to transform into CR 0 creatures. But, a baboon is a small creature while the tiny servant is, well, tiny. In true polymorph, emphasis mine:

You can turn an object into any kind of creature, as long as the creature's size is no larger than the object's size.

So, after 8 hours, the baboon will be too large, causing it to no longer be valid. Again, it may continue to be a baboon after 8 hours, since it was valid at the time of casting, but we don't know.

Answer 2

The GM makes a ruling

There are no rules for what happens when a target becomes invalid after a spell is already in effect. The designers say that having the effects be suppressed is a good rule of thumb, the rules' silence would normally mean effects just continue on unabated, which in practice works fine as a rule of thumb, and really it matters what specific spells we are talking about in what order for us to tell what, specifically, should happen and if either of those rulings will be problematic in a given case.

In this case, however, things are a little simpler. We know the spell tiny servant doesn't end and works fine when its target becomes invalid, because it belongs to that class of spells who always make their target(s) invalid. Since the game expects creatures not to be objects, most tables are going to rule that the creature created by tiny servant stops being an object (if not, that works too and is RAW compliant, and we're back to the general problem where everything is unspecified and the GM has to just make a ruling). Since it's no longer an object, it's no longer a valid target for itself.

True Polymorph, however, doesn't necessarily work when its target changes categories. The target is still a valid target, of course, but the rules provide just as much (I.e. No) support for the idea that spells' effects stop when a target becomes invalid as for the idea they should stop or change when a target becomes something that would have been affected differently if the spell were cast on it now rather than earlier, and the concepts are closely related. One imagines groups where effects end or are suppressed when a target becomes invalid will have True Polymorph react in some way when the tiny servant spell ends.