Very odd, because as I said I tested this after your post earlier and I have 4 extra slots created on my Ultimo4K usb stick.
It gets even odder
I did everything again.
Fresh flash od OBH 5.3.001. over everything in slot 1, just basic setup.
Then Open PLI 8.3 as a family image in slot 2 with complete setup of 14 LNB's and DVB-T2
Slot 3 PurE2 7.3 cause of some FNC plugins
These 3 are essential, other images are optional.
OBH 5.3.00
One create 4 more slots with red button
Then are followed Droid, VIX, ATV and last ver.OBH 5.3.
doubleO nine
And miracle - 009 in slot 7 create new slots
Summary - slot 1 is reserved for OBH 5.3.001. with no update.
I'll flash TeamBlue in slot 8 and that's all for today