After upgrade to 10.2.14 the red filled Circle Marker turned into blue hollow ring

Hi @calmforce and thanks for reporting this,
BUT I tested what you reported and all looks fine for me.
I cannot reproduce the Bug and the regression that you describe ... also applying your exact use case.

As you can see from the attached screenshots, my same local Leaflet View that reflects this Leaflet Demo one here:
https://www.geodemocracy.com/drupal_geofield_stack_demo/web/geoplaces-ma...

is correctly rendering circle markers that reflect the following options (and so on ... ):

{"radius":5,"color":"red","fillColor":"#f03","fillOpacity":0.5}

So eventually you just need to re-set / re-save your Leaflet View settings ...

It is true the latest 10.2.14 version made some changes in the Leaflet Circle Marker definition that are set and send to the js layer for final rendering, such the following:

https://git.drupalcode.org/project/leaflet/-/commit/b3f99b37dc933cb0610c...

but those would affect only you functional logic IF you are setting them in a hook_leaflet_views_feature_alter hook,
and YES in that case you should adjust your code and alter the

icon.circle_marker_options value

rather than the previous

marker.icon.options

Closing this as cannot be reproduced ...

(feel free to reopen if you really find evidence of this BUG and where I was clearly wrong in my review and statements)