Animation of the STAMP stops to work after long hold and release in Chrome IOS 15

### Description

Animation stops to work after long hold and release in the STAMP example in Chrome browser under IOS 15, AMP version 2106120107000

### Reproduction Steps

1) Make sure that you enable “Allow Cross-Website Tracking” feature in chrome.
2) launch Chrome in incognito mode.
3) Opt-in to 2106120107000 in cdn.ampproject.org/experiments.html
4) open in new tab https://www.gstatic.com/amphtml/stamp/qa/animation.html
5) observe result

Actual result: animation doesn't work or work incorrectly. You can see it in attached video

PS: under IOS 14.6 animation zoom works but story-ads animation doesn't work.
PSS: Other browsers aren't affected this issue.

https://user-images.githubusercontent.com/37351864/122594587-0cf55b00-d01c-11eb-8cfb-faeda2b6e2b7.MP4

### Relevant Logs

_No response_

### Browser(s) Affected

Chrome

### OS(s) Affected

IOS 15

### Device(s) Affected

Iphone Xr

### AMP Version Affected

2106120107000, 2106072053000