Long term stability of bluetooth connections
Bluetooth communication works with fixed time intervals that both communication partners have to agree during the advertising and connection phases. Such an agreement could be: "While connected, lets' meet every second. If one does not show up during 3 meetings, let's leave the connection state". 

From this you can see that timing is a very important factor in bluetooth communication. So both communication partners need a very accurate clock. No two clocks have exactly the same speed, one is always lagging the other. 

It could be that one or multiple of the clocks of your HC-05's do have too much clock drift, so the connection will be lost. Maximum clock deviation allowed for a bluetooth clock is ±40 ppm (i.e. ±40 parts-per-million!) over the entire temperature range. 

You could check this theory with multiple HC-05s. If individual modules lead to individual connection lengths, then this could be your problem.