Outlook 2019 - Inbox - subfolder problem

Windows 10 20H2 19042.662 (totally up to date)
Outlook 2019 Version 2011 Build 13426.20308 (totally up to date)
Inbox subfolder problem
I created a subfolder under Inbox called NTS, to no objection from the system. It is moderately populated - as relevant emails arrive I drag them there.
1) I noticed that NTS is not amenable to alphabetical sorting, it sits there at the top of the tree immediately under Inbox.
2) Right-clicking shows many of the options greyed out - I cannot rename this subfolder and I cannot delete it!
These actions are easily achieved for any other subfolder.
3) Right-click Properties shows no obvious distinction between this subfolder and any others.
Have I chosen a particularly problematic name in NTS?! This is all I can think of.
Regardless of cause, is there a cure? Ideally I would get rid of this and create a different subfolder with a new name.
Thank you!

- - - Updated - - -

Well - answering myself ("cure"; but NOT "cause", which I would still quite like to know .. ..)
Outlook is mimicked on my phone as an ExchangeActiveSync account. On this device (a) the alphabetical sorting of Inbox subfolders shows NTS where it should be (between M and O); and (b) (having emptied it as a preliminary) I was able to delete this subfolder conventionally and with no glitches.
When I returned to Outlook 2019 on W10, the problem subfolder had indeed disappeared.
So: a good result but as I say it would be interesting to know where the W10-based problem resided, and how it might have been addressed.