if this is not a bug - this has been changed back to the old behaviour in 5.10.

I can't find anything in CHANGES about the changes in behaviour. Both changes in behaviour are possibly side-effects of other changes. The effect of changes probably isn't tested against invalid inputs such as the one you presented.

Ref:
Changes up to 3.29

5.8.7  comes with version 3.05
5.8.8  comes with version 3.12
5.10.0 comes with version 3.2501
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