The glowing effect used to render around translucent pixels in textures, however this is no longer the case. This issue is likely a bug because this same bug has been fixed in the past (see 1.14 (correct) vs 19w39a (incorrect, same issue as now) vs 1.15/1.16 (correct) glowing effect).
This is could be because translucent alpha becomes opaque once it reaches the glowing shader rather than retaining partial transparency information as it always had before. The attached images compare 1.16.5 to 21w10a including checking the alpha values directly to confirm that partial transparency is not being rendered successfully.
Possibly worth nothing that the new transparency shader buffers (like itemEntity, translucent, etc.) now seem to render differently/incorrectly and have a blue background (see MC-218990) not sure if related or not.
Relates to MC-161887
To test:
1. Summon a glowing translucent entity
/summon minecraft:armor_stand ~ ~ ~ {ArmorItems:[{},{},{},{id:"white_stained_glass",Count:1b}],Glowing:1b}2. Notice that the translucent parts are no longer outlined with the glowing effect
Related issues
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pine1needle
onnowhere
MC-84121 is unrelated (that's not related to partial transparency/translucency but rather not highlighting cutout transparency correctly and/or not highlighting all layers of the entity), MC-173834 is also not related, that's full cutout transparency, not partial transparency as it should be highlighting all layers of the iron golem. MC-124109 seems to be related to partial transparency as horses have translucent pixels similar to here, and so this is probably up to Mojang to decide.
However, keep in mind that this partial transparency effect has always been the normal behavior since glowing was introduced. 19w39a was one case where it stopped rendering correctly, and then it was fixed afterword upon release of 1.15 so that partial transparency once again rendered.
The underlying issue is that the glowing buffer should be receiving transparency information as it always has. If the glowing shader itself wanted to ignore this information, then it could just consider alpha as 1.0 whenever greater than 0.0. (I would also say that partial transparency information in the glowing buffer is super useful for reading certain information from the game regarding a texture, and having it unable to be seen would be a detriment to that)
In the end, because behavior has changed from multiple previous versions, I would consider this a bug unless stated otherwise by Mojang directly.
onnowhere
This is the report that contributed to the 19w39a fix MC-161887
Felix Jones