This is very old but I just wanna throw this in in case somebody else has this problem, I found a “workaround” sort of.
Right click and add a set variable node for Jump Max Hold Time, but no need to connect it to anything. Select it, then in the details pane if you change the default value the change should take effect, at least it worked for me.
Obviously this isn’t ideal as it can’t be dynamically changed since the only option is to set the default value manually, but if your game only needs it at one value, it helps.
For some reason this setting and the JumpMaxCount is in the inherited parameters of the CharacterBP, but I honestly think it should be from CharacterMovement.