think the minimum offense level is the Tunnel Exit level - 6.
you might have a too low OL for those tunnels to start spawning OPs.
"low armies used to block tunnels" - FALSE... it was never true... they just don't spawn OPs based on your base... the only thing they blocked was maybe the spot for a higher level army that could've used/wanted that spot.
a helper... TE in range will have green or red markers on them when moving (or planning to move) a base:
https://raw.githubusercontent.com/netquik/CnCTA-SoO-SCRIPT-PACK/master/TA_Tunnel_Info.user.js
PS: it could be that some world settings are different... run this in browser Console to check if it's 6 or not:
ClientLib.Data.MainData.GetInstance().get_Server().get_POIActivationLevelDifference()