Horizontal and vertical gaps on some hyphen/equal based ligatures

I notice few bugs on ligatures involving equals and hyphens since version 4 :

• small gaps appear, most of the time between 2 first or 2 last characters
• a vertical gap occurs on specific equal-based ligatures like =>>= but maybe
  it's an expected behavior (related to issue #1043 ?), more or less visible depending on font size
• a visual glitch on =:= ligature (maybe related to the vertical gap problem?)

I guess it was introduced by the "Arbitrary-long combinative hyphen/equal based arrows" feature in v4. Arrows look good in v3.1 although they are limited in length.

I mainly notice the bug in Kitty terminal emulator on Arch linux.
It's less visible in other apps like L3afpad editor and not noticeable
in Firefox aside from the =:= glitch

Here is a screenshot @ 15pts font size for various weights, with Fira Code v5.1 / v4 / v3.1

Reproduce the reported ligature gaps in Kitty on Arch Linux using Fira Code v5.1, then compare the same examples across v4 and v3.1 as shown in the screenshot. Focus on the arbitrary-long combinative hyphen/equal-based arrows and the =:= ligature; done means the horizontal, vertical, and visual glitches are understood and corrected without regressing the v3.1 behavior.