App Insights Query with where clause on timestamp yields different results for every run

When I run the following query on app insights I get different results for every run

let allEvents=customEvents  
    | where timestamp between (datetime('2022-12-30')..datetime('2022-12-31'));  
allEvents  
| where name == "Event1"  
| project st1 = timestamp,user_Id  
| join kind=innerunique (  
    allEvents  
    | where name == "Event2"  
    | project st2= timestamp,user_Id  
    ) on user_Id  
| where st1 < st2  
| summarize count()  

I few further observations

• If I remove the where clause, the results remain constant
• If I use join kind = inner, the results remain constant