I'm not sure, I had the same happen to me, but never after I raised the bid.
You say it worked the last 4 times, that's 4000gp, if it was tie each time, then 5'th bid may had been at 2000gp, or you lost it at 6'th bid, as there was no gold left
I had a gamemaster checking my lost community vendor, he did not find anything, but sure there can still be a bug.
Merchants wishing to lease a stall continuously for more than 7 days may place some funds in reserve with the Bazaar Authority to match the top bid on that stall. When their lease ends and the stall is re-auctioned, if the top bid is not higher than the match funds reserved for that stall, then the current tenant gains a new 7-day lease on the stall but the amount of the top bid is deducted from the reserved match funds. The top bid remains in effect for future auctions. A merchant’s bid matching information is kept private.