Welcome to the forum.
I've noticed similar on the flysheet of an ancient Vango Force 10 that I sold a couple of months back - I suspect that it is the PU proofing disintegrating, which I believe it does with time, no matter how well the tent is stored. Easy to re-proof a flysheet, but not so sure that you would get the total waterproofing required for a groundsheet.
My 1990s Wild Country Trisar is not suffering from this problem though - slept in it last night, in fact - the groundsheet is still perfect.
You could test it - erect the tent and pour some water into it - maybe put a shallow pan under the groundsheet and pour the water onto the groundsheet above that, to see if any leaks through.