The hot bar will only allow loading one instance of a bucket (empty, lava or water. milk untested).

Recreation:
Starting with inventory contents as in Shot1'

• Inv Slot6 - Lava Bucket
• Inv Slot7 - Lava Bucket
• Inv Slot8 - Water Bucket
• Inv Slot9 - Water Bucket

Lava:

• Select focus on HotBar Slot6, tap Inv Slot6 - Lava bucket from Inv Slot6 populates HotBar Slot6.
• Select focus on HotBar Slot7, tap Inv Slot7 - Lava bucket from Inv Slot7 or possibly Inv Slot6 (can't tell which) populates HotBar Slot7, HotBar Slot6 populates with whatever was in HotBar Slot7 previously - Shot2'
  

Water:

• Select focus on HotBar Slot8, tap Inv Slot8 - Water bucket from Inv Slot8 populates HotBar Slot8. - Shot3'
  
• Select focus on HotBar Slot9 - Shot4'
  
• Tap Inv Slot9 - Water bucket from Inv Slot9 or possibly Inv Slot8 (can't tell which) populates HotBar Slot9, HotBar Slot8 populates with whatever was in HotBar Slot9 previously - Shot5'
  

Same behavour for non stacked empty buckets.

Previously this was ok as I have used many water buckets in hotbar slots while building farms often.