Can you post some pictures of the bottom of the feet of the birds in question? Some bumblefoot lesions come out mostly intact and resolve easily, some do not. Any kind of wound, scratch, cut, splinter, etc, can become bumblefoot if bacteria gets in there. It's almost impossible to prevent it completely, especially if your birds are ranging. It can happen stepping on something, jumping up or down, scratching around, etc. Since you are having an issue, I think I would just start doing foot checks regularly, so hopefully you find them early, when they are a little easier to treat usually.
Actual surgery is not my first choice, I usually start with this method:
https://www.tillysnest.com/2015/12/non-surgical-bumblefoot-treatment.html/?spref=pi
The important part is to remove all of the infection, if any is left in there then it will regenerate. When you do a removal, set a block of time, take your time, and flush it out really well before packing it with ointment and bandaging. Reapply ointment and change the bandage daily until you know it's healing well. Sometimes you do have to do repeat cleanings to get it all out, every foot is different. There is one other treatment option of daily soaks with tricide neo, but it can take quite some time, so it's a commitment. I have found it most effective on smaller lesions that are more superficial.
https://ouroneacrefarm.com/2013/11/09/bumblefoot-treatment-tricideneo/
If digging is required to get it all out, I use a curette like this (you can get on amazon):
Actual surgery is not my first choice, I usually start with this method:
https://www.tillysnest.com/2015/12/non-surgical-bumblefoot-treatment.html/?spref=pi
The important part is to remove all of the infection, if any is left in there then it will regenerate. When you do a removal, set a block of time, take your time, and flush it out really well before packing it with ointment and bandaging. Reapply ointment and change the bandage daily until you know it's healing well. Sometimes you do have to do repeat cleanings to get it all out, every foot is different. There is one other treatment option of daily soaks with tricide neo, but it can take quite some time, so it's a commitment. I have found it most effective on smaller lesions that are more superficial.
https://ouroneacrefarm.com/2013/11/09/bumblefoot-treatment-tricideneo/
If digging is required to get it all out, I use a curette like this (you can get on amazon):
