Tunnel goes yellow for several minutes

A client tunnel pool goes 'yellow' in the console sidebar for several seconds or minutes, then recovers. Seen this periodically for several months (years?) but with the network currently in pretty good shape it's clear this is a bug, not tunnel build failures.

Just seen on a router with generally 80+% client build success, and was almost 100% at the time. Tunnel configured for 2 in/out tunnels. Checked client LS tab, client leaseset was there but with only one lease and it was expired. A minute later, there was no leaseset there any more as the expire job had taken it out. After a few minutes, tunnels were built, LS was there again, tunnel went green. Client build success stat stayed above 90%.

This could be either failure to request LS from client or failure to build tunnels. The LS with a single expired lease is a clue that it's probably the latter, we aren't building tunnels, until some failsafe kicks in? Suspected culprit is BuildRequestor. To be researched further.