weird interaction between redraw and echom

Dear vim users,

TL;DR: during sourcing of scripts 'echom' messages sometimes display and
sometimes don't. Putting 'redraw' somewhere during the sourcing process makes
them appear deterministically. Why they don't show up consistently?

--

I was writing a function to help me debug one of my plugins and it took me a
lot of hours to finally narrow down what was causing my issue.

By toggling a variable that defines a debug flag some messages were supposed to
be 'echom' to the screen. However they did not. I was struggling with this
random behavior trying thousands of combinations until I finally got a
consistent behavior by putting redraw in just before some of the 'echom'
messages.

Why is that the case?

Best,

--
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