Inner groove distortion as tonearm moves toward the center of record What is cause??

Can't put it any better than Mike L. A tangential tracker does make a significant improvement but there is still the slow speed which limits high frequency performance. This is where 45 RPM records shine. 

It's just physics. 

https://www.yoursoundmatters.com/vinyl-record-inner-groove-distortion-simple-explanation/

gdnrbob +1, I'm with ya!

In all of my experience with vinyl I don’t recall any issues with end of side distortion. Not even during the times of chronic audiophilia, not even with those extra long Bob Dylan LPs that couldn’t fit onto one side of a C90 cassette.

Even in the days of my Rega 3 with its silver S shaped arm I can’t recall any issues. Certainly not with the Ittok arm on my LP12. I think I would have done if there had been any as the final track on many of my favourite Beatles and Dylan LPs would be a standout one, played regularly.

I should add that I was careful with both cartridge alignment (gauges, mirror etc) and only ever used stylii which featured an elliptical profile. Back then we were told that a conical stylus could never track as well, so that was a big no-no.

Also the cartridges I used (Nagoaka and the better moving magnet Linn’s) were fairly easy to align. Tracking weight was always set within manufacturer’s given range, and finalised by listening carefully.

I also had an electronic Audio Technica stylus cleaner with its vibrating pad back then, which I can’t believe is not available today. It was pretty inexpensive but very effective, sadly I gave it away when I sold my Linn.

I remember doing tests even with a perfectly aligned Supex SD1000 in DV505 on Linn LP12, using 3 special CBS test records they sent us. 

I could even see on the oscilloscope with one of these CBS records of which one whole side is number of tracks all cut with the same amplitude 1khz sine waves, just so you could measure cartridge alignment distortion, the inner most groves had visually the worst "more" distortion than the others and the outer one was next worst, and if you can see it with your eye, believe me it's bad.
After that I was aware of it much more when playing music from then on, which really started to piss me off. Then thanks for a tangential arm Sony PSX800/XL55 Pro TT and MC I was much happier, then all changed again thanks to digital for coming along. 

Cheers George