To recreate:
/effect give @s minecraft:health_boost 1000000 255 true
/attribute @s minecraft:generic.max_health modifier add 0-0-0-0-1 name -25 add
/effect give @s minecraft:wither 1 1 true
Note: This should remove 25 health points but instead only removes 5. Health Boost amplifier 250 also gives the same amount of health as amplifier 255 (1024 health points), but works fine. It seems that the /attribute command takes the extra 5 (amplifier) * 4 (Health Boost hearts) health into account, even though the hearts aren't actually there. In summary, the /attribute command thinks that the player has 1044 health points when given Health Boost with an amplifier of 255.
Linked issues
Comments 2
Thank you for your report!
We're tracking this issue as MC-10755, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as Won't Fix, which means this is considered a bug but won't be fixed. The description of that ticket or the comments might explain the rationale. Please do not leave a comment on the linked ticket.
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Duplicate of MC-10755, unsupported amplifier.