I don’t understand the second part of what you said. The Purity Seal can get the Enchant for All Resist or All Resist when Protected, but if you have one then you won’t be able to roll the other. Having one enchant makes the other unavailable. Is this a bug or intended?

On other gear, you can have Damage Reduction and Damage Reduction when Protected on the same gear. You can also have Critical Hit Strength and Critical Damage on the same gear. You can have Heat and Physical Resistance on the same gear. It is only Purity Seals not allowing that one specific enchant combination, I have not found a similar issue with any other gear/enchant combination (having one enchant makes another enchant unavailable).

I was wondering about that too. Character sheet usually shows you hit the cap by making the number yellow, like when you get 60% warp resist. But suppression damage reduction seems capped at 75% even though the number is still white.

And noticed the same thing with armor. You can have damage reduction AND damage reduction while protected on the same armor, but can only have all resist OR all resist when protected on the same armor. Seems like a bug to me; if you can have both damage reduction enchants, then you should be able to have both all resist enchants.