Of course there are things they can do without codes, just this dealer is very lazy, does not want to comit time to true diagnostics and may njot have truly talented and certified machanics.
Codes are a tool, not the answer.
Things it can be, a worn Inhibitor switch (not always telling the car what the selector is in), a bad motor mount, a bad cable or cable out of adjustment, or a valve body issue.
What is happening is for some reason the car thinks you have decided to hold 2nd gear. Every modern trannywill force an upshift at redline even when the operator decides to hold a gear. Netural after a bump is pointing to the gear selector cable/mechanisim being out of adjustment.
This may be several issues not just one.