I have a party that's about 75% of the way through its current level (I checked just before starting the Long Rest). At the end of the rest, when I clicked "Close", I got a pop-up warning entitled "Skipping Character Level Up", asking if I was sure I wanted to exit. None of the characters were shown as due a level up in the Long Rest window (under "Attune" and "Prepare Spells"). Whatever programming logic you are using to trigger that warning popup, it needs to be made consistent with the logic that displayed "Level Up" in the Long Rest window.
Addendum:
The above problem happened during a player-created campaign. After I pressed submitted the bug report, and tried to exit the Long Rest window in-game, the screen suddenly displayed a message telling me that my party was being attacked by Soraks, then the Long Rest window was redisplayed, this time with Level Up options.
After I did the level upping, and closed the window, the combat log showed that my party had been granted 2318 Exp Points (immediately after the Long Rest's list of crafting successes and failures). It didn't say whatthe XP was for, but several minutes earlier in the game, an NPC made a big issue out of my party taking a rest. It was inside a castle, after killing all of its baddies. I suspect the campaign writer was awarding XP for the Long Rest as an easy way of rewarding success for clearing the castle.
The bug is still an issue, but it seems very much a corner case.