This is not a CoreELEC problem.
It could be related Kodi/Skin/Subtitle font you chose.
Test it with same settings (kodi version, skin and subtitle font) on Android or another system and see if it’s still reprodcued.
i had the same problem with Netflix addon…no luck changing font… but after the Kodi update to 18.6 in CoreELEC nightly build this problem with subtitles disappeared
Same add-on works very well in android & windows with KODI 18.6.
Netflix Add-On producer said it may be related with KODI or streaming addon.
Some people said COREELEC nightly version works well.(It base on KODI 18.6)
KODI 18.6 version works well in every OS but 18.5 version show this phenomenon.