DateTime format with OH3 items not working

[rossko57]

@denisdmenace

the Profile of Timestamp on the channel is stubbornly overriding & ignoring

The channel linking process has a feature (in my view poorly designed) where the binding can ‘suggest’ - in practice, silently force - a suitable pattern into metadata.
That should happen only once, at linking time.
You should be able to hand-edit a more suitable pattern after that one-time action.

Clearly that can lead to an issue where a switch type channel is linked to a DateTime Item, and a ‘switch’ suitable pattern is forced on the DateTime.
Open issue -

Switch item with linked channel and hysteresis profile leads to error on the item

Unrelated to that -
The timestamp profile itself had a weird bug where it kept forcing Item update if something else changed it - such as you might get when linking two timestamps to same Item.
That one at least has been fixed recently -

[profiles] timestamp-update channel profile forces Item state

but I wonder if an unnoticed effect of that was also re-forcing target Item pattern.

EDIT - ah I was just reminded of a nasty consequence of the ‘link forcing a pattern’ strategy. A binding may also suggest ‘state option’ metadata. e.g. a motion sensor switch sets [ON = motion, OFF=idle]. That is really going to screw up when used with a DateTime. You might check your misbehaving Item(s) for that metadata, too.