Commented by: @hvlad

Below is typical picture at false deadlock detection time

a) Owners, participating in deadlock (backtrace, head of chain marked by 0) :
1 deadlock chain: OWNER BLOCK 377540 Process id: 2968 Flags: 0x04
2 deadlock chain: OWNER BLOCK 396488 Process id: 2748 Flags: 0x04
1 deadlock chain: OWNER BLOCK 377540 Process id: 2968 Flags: 0x04
0 deadlock chain: OWNER BLOCK 401656 Process id: 3024 Flags: 0x04

b) Two locks on which owners are waiting :

LOCK BLOCK 948936
Series: 3, Parent: 346628, State: 6, size: 8 length: 8 data: 0
Key: 0001:009782, Flags: 0x00, Pending request count: 5
Hash que (3): forward: 1006312, backward: 366144
Requests (6): forward: 1375776, backward: 1109580
2 Request 1375776, Owner: 396488, State: 6 (6), Flags: 0x00

	Request 876536, Owner: 532236, State: 0 \(6\), Flags: 0x02
	Request 1117016, Owner: 677976, State: 0 \(6\), Flags: 0x02
	Request 1400188, Owner: 544104, State: 0 \(6\), Flags: 0x02

1 Request 1366912, Owner: 377540, State: 0 (6), Flags: 0x22
Request 1109580, Owner: 508804, State: 0 (6), Flags: 0x02

LOCK BLOCK 364744
Series: 3, Parent: 346628, State: 3, size: 8 length: 8 data: 0
Key: 0001:007839, Flags: 0x00, Pending request count: 4
Hash que (5): forward: 998428, backward: 6564
Requests (9): forward: 1409704, backward: 1230128
1 Request 1409704, Owner: 377540, State: 3 (3), Flags: 0x100
Request 1391660, Owner: 532236, State: 3 (3), Flags: 0x100
Request 1175112, Owner: 677976, State: 3 (3), Flags: 0x100
Request 1339992, Owner: 544104, State: 3 (3), Flags: 0x100
Request 875028, Owner: 508804, State: 3 (3), Flags: 0x100

	Request 1403776, Owner: 588040, State: 0 \(6\), Flags: 0x02
	Request 1399668, Owner: 486280, State: 0 \(3\), Flags: 0x02
	Request 1114520, Owner: 627348, State: 0 \(6\), Flags: 0x02

2 Request 1230128, Owner: 396488, State: 0 (6), Flags: 0x22

As you may see request 1375776 (of owner 396488) just granted a lock 948936. This request blocked some other requests, one of them by owner 377540. Note, request 1375776 have flags 0, i.e. it was not marked as blocking and not seen blocking notification. Therefore it have no chance to release the lock to break circle in wait-for graph.

LOCK\post_pending() routine wake up owner of first blocked request. If another process run deadlock scan before blocked owner awakens (and calls post_blockage) it reports false deadlock.