Question:
Why does the display order of Multichannel Activity records on the Account Timeline page change when refreshing the browser?
Answer:
The Multichannel Activity records displaying on the Account Timeline page are sorted by Start_DateTime_vod_c in Descending order. If the Multichannel Activity records have the same Start_DateTime_vod_c value, the order is variable.
Currently, this is working as designed.

Related Documentation:
CRM Help Documentation: Multichannel Activities on the Account Timeline