Flagging an activity retains the strain score but removes signature calculation.
Xert normally ignores power spikes during analysis. However, you can enable Max Power on the Activity Table, sort by that column, and flag any suspect entries there.
If you still question the results, post a screen shot of your XPMC chart set to 3 Months or Year.
To select a custom date range for the XPMC progression chart, set filter to Forever, then place mouse pointer on start date you want (date shown at bottom) and drag highlight to end date or Today line.
When viewing the progression chart after activity edits it’s a good idea to refresh browser page to clear the cache.
Welcome aboard. 
Lots of newbie tips in my Onboarding post –
Onboarding steps for Xert Trial users and newbies - General - Xert Community Forum (xertonline.com)