/bossbar add a 1
/bossbar set a value 0
/bossbar set a players @a
/execute store result bossbar a max run data get entity @p a
/bossbar set a max 10
/bossbar set a value 5

A bossbar max is set to zero, should in code: bossbar max > 0 ? bossbar max : 1. Code debugged as 0 / 0 = NaN (bossbar value / bossbar max = return value)