Great timing Beth as we have been battling with exactly this issue this week, both for a client build and for a tender that requires the same capability (it's a MUST HAVE feature, so we will probably lose it if we don't have a workaround). Like you, we have considered a Flow, but it's only a partial solution.
I got terribly excited when looking at News this week, thinking it was possible, only to discover, as you did, that useful filtering disappeared when more than one source is selected.
So much good stuff, to be tripped up by things like this.
What's the latest on this https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Ftechcommunity.microsoft.com%2Ft5%2Fuser%2Fviewprofilepage%2Fuser-id%2F96&data=01%7C01%7CSimon.Hudson%40cloud2.co.uk%7Cd227138a9a2e4b447c2808d62e22148c%7Cd99524016be4413d9f7a3b863e27ef1f%7C1&sdata=RrdScHWyGFK75GhWDyy3A%2BY6bq9QQdzgKIos9%2BCIAYQ%3D&reserved=0? It's been an issue for quite a while now and is really hurting the Migration to Modern story with Enterprise customer, alongside the similarly limited search experience