The solicitor is not being difficult. If a solicitor spends money, then his\her own financial records will show the outlay incurred. It is unthinkable (because it would be the stupidest fraud ever) for a solicitor to claim he\she had paid probate office fees if in fact the fees had not been paid.
Some solicitors scan in everything. Some don't. The receipt which issues from the probate office is a hand-written receipt on a piece of paper the size of a compliment slip - a likely candidate for not being scanned in. If papers are lodged by post, the receipt may or may not make its way out to the solicitor (but if lodged by hand, it will be received on the spot). Also, none of this unduly matters because the probate fees paid are inserted on the grant of probate when issued.
Nothing happens in a vacuum. If the OP is asking for a receipt, it is because they want to verify. There is nothing wrong with that except for one thing: the solicitor almost certainly did not price his or her work on this basis.
But now, to get reimbursed for an outlay which was properly advised in advance in writing, the solicitor (who may be remote working) may have to get someone to dig out the paper file, copy the paper (if they even have it) and send it to the client. Or he\she can take roughly the same amount of time explaining this to the client.
Either way, it is adding unbudgeted time and cost to a file at a time when many offices are under immense resource pressure.
But, unfortunately, if the solicitor doesn't do this, they will be leaving themselves open; they may be accused of being difficult.
The lesson for the solicitor is better client communication up front.
Add yet another paragraph into the standard engagement letter about what will be provided and when OR implement (and charge for) enhanced service levels to include copying all receipts and sending them to the client as and when each outlay is incurred.