The only 'easy' fix for this, and actually the 'right' fix, is to use a different SSID per band.
CORP = 5GHz only
CORP Legacy = 2.4GHz only
Band-Steering is a feature that works because the AP will essentially ignore X number of probe requests on the 2.4GHz radio (3 i think), with the hope that within that time frame the client will just associate to the 5GHz radio and call it a day.
You have to keep in mind though that band-steering is only a 'persuasion or suggestion' to the client, it can't actually force or make the client connect to its 5GHz radio. The client is 100% in charge of which radio he will want to connect to. And as you've seen, band-steering can cause issues.
The fix again is to separate the SSID between both bands, and this will prevent clients from having the issue's your talking about.
But I also agree with @PhilipDAth that another AP on the floor above is what you should do, although I understand that you can't. Sometimes though the solution doesn't have to be perfect. You could simple place the AP on someone's desk since there is already ethernet cables ran to those locations for example, and that would at least be better than nothing (hopefully).