Ulduar raid instance not swapping/staying set to Heroic

Legacy raid difficulty not swapping/staying set to heroic or 25man. Everytime I zone in to Ulduar, it keeps setting the raid instance to 10 man Normal Ulduar. Even though I have it set to 25 man Heroic prior to entering the instance, i’ve tried rezoning over checking it and swapping the setting, and it keeps setting it to normal.

Is this a known issue or some sort of undocumented change?

Last night before reset was having this same issue, went ahead and killed some bosses just to see and it was only dropping normal loot. Trying to farm heroic for legendary stuff, xmog, and Mimrons head.

Ulduar does not have a heroic mode nor a separate 10/25 man version. It only has a flex mode (10 to 30 players) now that combined the 10/25 man loot tables.

The changes were made to it when it was added to the list of Timewalking raids in patch 7.3.5.

More info in the links below:

Patch notes: https://worldofwarcraft.com/en-us/news/21365423/world-of-warcraft-735-patch-notes

Wowhead article: https://www.wowhead.com/news=277715/ulduar-changes-on-patch-7-3-5-ptr

Wowhead article: https://www.wowhead.com/news=278262/more-details-on-ulduar-in-patch-7-3-5-loot-transmog-and-mounts