With a quick search on the Internet, I found that this could be because Core is not completely sync but it's:
{
"version": 220000,
"blocks": 809706,
"headers": 809706,
"verificationprogress": 0.9999983770795251,
....
Checking mempool.space 809706 is the last block.
is just estimation and one of Bitcoin Core member said there's no problem if it's slightly below 1.0[1]. It's proven by 0.9999983770795251 doesn't refer to total of verified block since the value equal to 6249989856747 / 6250000000000 [2].