I'm not sure the bank has that much to do with it.

Commercial real estate valuation is based entirely on its ability to produce income. Lower the rent, lower the value. And that's a problem because most commercial leases are long (5-20+ years) so you're locking in an asset writedown for a long period of time. So it can be better to leave it vacant and pretend the value hasn't changed.

You can still run into problems with this (eg servicing the loan). So I don't think it's quite the issue that banks have to approve lowering the rent so much as the owner might lower their asset value and have problems with the LTV and DSCR so the bank may then require you to refinance or add capital.

By the way, we've gone through this before. Up until the 1990s, law firms were by far the largest tenants of office space because they had very large law libraries. Then that went online and they downsized. This was an acpolaypse in the 2000s combined with the dot-com bust.

I think the lag you're talking about is on banks essentially foreclosing on a building and selling it off, allowing the new owners to charge less because they paid less.