Encountered several times, with normal mobs (no bosses with legendary resistances here).
Spells like sleet storm or grease, mob walks in the area, fails dexterity skill check and falls prone.... and immediately get up and continue walking as if nothing happened. Without any other skill check, and of course in the same turn.
Happened too with entangle and dark tentacles, in those cases failing strength checks and same issue, they continue walking as if nothing happened -- with nothing else in the combat log, just the failed skill check -- it feels like downright cheating. It doesn't happen in every case but pretty often.
Some mobs are immune to the prone and/or entangled conditions. When you have 4/4 knowledge of the creature, check the immunities.
Example: https://solastacrownofthemagister.fandom.com/wiki/Pilgrim, immune to Prone.
Example: https://solastacrownofthemagister.fandom.com/wiki/Spectral_Avenger, immune to Necrotic, Poison, Cold, Charmed, Exhausted, Frightened, Grappled, Paralyzed, Petrified, Poisoned, Restrained, Prone, Hindered
Thank you for your reply; but that was a bug report -- if you noticed what I explained these are mobs that FAIL their saving throw against that status, when a mob is immune to a condition or type of damage it is clearly stated in the combat log, and of course we check the bestiary for immunities in each mob. (It is a co-op run).
See this image as a example:
https://steamuserimages-a.akamaihd.net/ugc/1856060164414877178/45FE7BF726DF6EE5582093D5948D362F90604B46/?imw=5000&imh=5000&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false
The dominion arcanist in the image fell prone after that failed check, and then got up with NO further checks as you can see in the combat log, and directly walked out of the range of sleet storm to the position you see in the image. We have more cases. There is no explication for this.
Gotta say I'm confused by this too and as ekarinae says above (we're playing co-op), we've seen it with multiple encounters. The mob fails the check, falls and then just gets up and continues to move/attack as if nothing had happened.
re: Sleet Storm
Note that falling prone does not end your movement and standing-up from a prone position just consumes 1/2 of your movement speed so a creature with a speed of 6 (squares) could start their turn in a Sleet Storm, fail their DEX check and fall prone, then stand-up and move 3 squares and still have an action.