Cactus flowers are stated to be able to be placed on any blocks that can give central top support.
However, it seems leaf blocks are an exception and cannot support cactus flowers.
Linked issues
Comments 4
Glass was reported to me by another user, but yeah I can't reproduce it either.
Will edit the bug report to specify it as leaf blocks being the main issue
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Can't reproduce with glass. Cactus flowers can be placed on all blocks that provide central top support, which includes glass, stained glass, tinted glass, glass panes and stained glass panes. Can confirm with leaves-