You are setting the value for each row so when you place this in the group footer it is showing your the value (blank) from the last row in the group.
You can either do a MAX value on that, or add 2 more formulas to manage it and reset on the group footer level or use a running total. I perfer RTs.
You can use MIN or MAX as needed depending on which you want to see. I am assuming MAX time value here
RunningTotal
Name=MAX_ArriveTime
Field to Summarize={ED_IEV_EVENT_INFO.EVENT_TIME}
Type = Maximum
Evaluate= Use a formula
{ED_IEV_EVENT_INFO.EVENT_TYPE}='50'
Reset = on change of group (select Event_Type)
Place in Event_Type group footer
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