@Aussie
There is probably a special rule regarding rolling over a Roth 401k to a Roth IRA. A better expert on this board is @dmertz ; they can explain whatever I have forgotten or gotten wrong.
Now there is a different pro rata rule that I do know a little about. If you have pre-tax money and after-tax money mixed in a single 401k account, when you roll that over, the custodian is supposed to put the pre-tax money in a traditional IRA and the after-tax money into a Roth IRA. But you said you had a Roth 401k account. So unless I am missing something in your explanation, this second situation does not apply to you.
Going back to the rollover of a Roth 401k to a Roth IRA, this article says the money keeps its character -- your workplace contributions go into the contributions bucket in your IRA, and the earnings go into the earnings bucket.
https://irahelp.com/roth-401k-to-a-roth-ira-rollover-how-does-this-work/
Assuming that is true, you need to add up your contributions over the years and subtract your withdrawals that you said you have already made (since withdrawals come from contributions first). That number might or might not come out the same as Fidelity calculated (I would try and calculate it independently to see if you match their results). If it is the case that, out of $100,000 rolled over, $49,000 is in the contributions bucket, then the first $49,000 you withdraw counts as contributions and is tax-free, and if you withdraw more, that counts as earnings and is subject to tax if it is before 2030.
My thought that the entire amount counts as a rollover or conversion is apparently wrong. But best to wait for @dmertz.
