What is the problem? Is there any Lua errors?
No.

Is this issue for Classic, TBC or Retail?
TBC, but when I played Retail it occurred there to.

What steps will reproduce the problem?
Activate at least 2 different units (e.g. Nameplate and Arena).
Select different icon sizes for them (e.g. Nameplate = 20, Arena = 40).
Use "Toggle Test Mode" a few times and you'll notice that the icons for both Arena and Nameplate are (seemingly) randomly chosen between 40 and 20. So, for (e.g.) Arena1 you'll have for example 2 icons that are 40, and 1 icon that is 20.

Note: The issue seems to be happening only for Nameplate and Arena, other frame icons seem unaffected (I set them to 40).

Does the issue still occur when all addons except Diminish are disabled?
No. The issue only occurs when Masque and Diminish are used together. Since Diminish is intended to support Masque I thought it was worth reporting.

Was it working in a previous version? If yes, which was the last good one?
Not to my knowledge (TBC).

Any additional info? If you play on a non-english client, please include your locale type:
English client.