export
is a declaration utility. Those of its arguments that resemble variable assignments are expanded the same way variable assignments are, i.e neither pathname expansion, nor word splitting is performed on them, and the value part is subjected to tilde expansion. Although a bug report was made in 2010, even the most recent edition of the standard fails to document this behavior. However, the changes suggested here were applied in the 202x.1 draft (see Austin Group homepage if you want to obtain a copy), so it is very likely that when the next edition of the standard is published, under Simple Commands, following the first sentence of step two, you will see the statement below that mandates the behavior you deemed strange.
If the command name is recognized as a declaration utility, then any remaining words that would be recognized as a variable assignment in isolation shall be expanded as a variable assignment.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…