This was tested with v2.3.8 but there were no changes to it in the last 2-3 versions.

Currently all wotlk classic raids fall under Other as defined in statuses/Status.lua:19. This makes load conditions based on instance type useless for raids.

GetInstanceInfo() returns the following difficulty IDs for wotlk classic raids
Ulduar (and P1 raids I suppose): 10man 175 | 25man 176
ToGC (and ICC in the future): 10man 3 (normal), 5 (heroic ) | 25man 4 (normal), 6 (heroic)

Normal & heroic dungeons seem to function fine and alpha/beta dungeons just reuse the DifficultyID of heroic dungeons (2)
Any chance for a workaround for this?

Grid2 does not differenciate between normal/heroic raids even in retail because i think it is not necessary.
For example disabling a heroic debuff for a normal raid is not necessary because anyway the debuff will never will be displayed in the normal raid.
And if you want to do something different with some debuff shared between normal and heroic, heroic debuffs have different spellIDs than normal debuffs so you can create the heroic debuff using the spell identifier/number.

Yes, filtering based on normal/heroic raids isn't useful, I agree. My worry was with raids being classified as Other, but it looks like only raids are part of that category for wotlk classic, so it can be treated as filter for raids only. I will close this ticket.