Possible bug in L/R pan automation when using preview/fill

I've got this thing happening with the LR panning of a stereo track when I use preview and fill range to write a pan value. Mono tracks function as expected, it's only on stereo tracks. It only happens when using the preview/fill automation way of writing. If I write in realtime it works as expected, but if I use preview, after filling the new pan value to the range the left side of the stereo track is positioned in the centre instead of hard left, for not just the range I filled, but for the whole timeline, and I can't undo it or adjust it back to hard left with any pan controls ie spread..
I can work around but it just means I can't loop a range to position the pan in realtime then fill that range.
The attached is what I get after using fill automation to a range in preview automation mode.

DaVinci Resolve - PA test 2 24_04_2021 9_50_53 AM.png (30.06 KiB) Viewed 555 times

Is automation data present or is virgin data written across that Range only?

Hi Reynaud,
previous automation is present, so the fill range is overwriting the section.
I'll do a write to the whole length of the timeline with the pan in it's central position before
I start writing new sections. It seems to be related to using the preview automation mode.
If I don't switch on preview and instead just hightlight a range, move the pan and fill range it works
normally, but of course I can't position it with the pictures running doing it that way, or audition the new position.
It's only the L/R pan that does it, other controls like spread and rotation seem to work fine.
Thanks