Take a look at this post:
I suffered this bug when I joined in 2013 and again when I rejoined in 2017.
I notice that the issue crops up on a regular basis on the help & support forums.
Anyone know why this issue can't be resolved once and for all?
aint got a clue but as you say its been a pain for ages
why not add your vote to this idea over on labs @studio_muppet
Simple answer: if you're intending to use the full value of a topup to buy a goodybag, do not top up separately,