Can a rider with both mounted combatant feat and saddle of the cavalier be forceably dismounted by attack actions?

Question

If a rider was successfully grappled by an opposing threat, who had both mounted combatant feat and saddle of the cavalier would this dismount them from their steed?

Context: my character with both mounted combatant and saddle of the cavalier was attacked by a huge dragon, which could consume foes whole if successfully grappled and were at least Medium in size. I argued I could not be consumed while on my steed as while on my steed, the steed and I would be one large creature (warhorse - find steed). However, my DM ruled I could be dismounted from my steed if I was successfully grappled stating that saddle of the cavalier did not protect against attacks being made at me for being dismounted, only my steed. I disagreed as nowhere does the saddle of the cavalier state it only protects against attacks made at your steed that could dismount you, however respectfully followed the DM's ruling.

Saddle of the Cavalier states:

While in this saddle on a mount, you can't be dismounted against your will if you're conscious, and attack rolls against the mount have disadvantage.

And mounted combatant says:

While you are mounted and aren't incapacitated… you can force an attack targeted at your mount to target you instead.

Answer 1

I answered this question as part of an answer to a similar question.

"Dismount" is a game term defined in the rules for mounting and dismounting.

Here are the things from the mount rules that the Saddle of the Cavalier prevents:

Mounting and Dismounting

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[...]

If an effect moves your mount against its will while you’re on it, you must succeed on a DC 10 Dexterity saving throw or fall off the mount, landing prone in a space within 5 feet of it. If you’re knocked prone while mounted, you must make the same saving throw.

If your mount is knocked prone, you can use your reaction to dismount it as it falls and land on your feet. Otherwise, you are dismounted and fall prone in a space within 5 feet it.

This is what the magic item description is referring to when it says "you can't be dismounted against your will". The idea here is that the rules define for us what it means to be dismounted against your will.

Getting swallowed is different from being dismounted.

Being swallowed is not covered in the rules for mounting and dismounting, because dismounting is a specific thing defined in those rules. The Saddle of the Cavalier will not prevent being swallowed, because it is not the same thing as being dismounted.

To me, this makes narrative sense. Being really good at riding a horse isn't going to prevent you from being removed from your horse by a giant dragon that has you in its jaws.