Inconsistent rendering of Thai characters

When rendering Thai non-spacing marker (NSM) characters - glyphs that must be combined with other characters, the rendering is inconsistent across platforms with regards to the fallback font.

In a usual situation we don't want to render NSM without a glyph to combine with but sometimes we do need to manually paint a widget or some other task that requires a slightly lower level manipulation of how glyphs are laid out. For example, an on screen Thai keyboard.

The fallback font renderer is inserting a circle (Unicode codepoint U+25CC) for NSM and Thai character SARA AM U+0E33 [see note below]. However, this is not consistent as explained:

Action: set widget to font and set text to a NSM codepoint

Expected behaviour: render most of the glyph outside of widget because it should be combined with a non NSM glyph

Actual behaviour:
Mac - insert circle if rendering with fallback font but not for Thai fonts
Windows - never insert circle for Thai fonts or fallback font (EXPECTED)
Linux - insert circle for both Thai and fallback fonts

It looks bad if you attempt to insert your own circle for combining and aren't aware of this issue.
[Note] SARA AM is not a NSM but must be combined with another glyph so treating it as NSM for the purposes of deciding whether to insert the circle is consistent.

I attach a small program that can be used to explore this behaviour. Select a codepoint labelled as NSM and then change the font.