context.isMaster property is true across multiple amp-ad tags frames

### Description

Trying to use isMaster to determine main thread to reuse resources properly, but seem like this property is set true in all frames. Am I missing smth?

### Reproduction Steps

I used this example [playground](https://playground.amp.dev/#share=PCFkb2N0eXBlIGh0bWw+CjxodG1sIOKaoT4KPGhlYWQ+CiAgPG1ldGEgY2hhcnNldD0idXRmLTgiPgogIDx0aXRsZT5hbXAtYWQgLSBFeGFtcGxlIDE8L3RpdGxlPgogIDxzY3JpcHQgYXN5bmMgc3JjPSJodHRwczovL2Nkbi5hbXBwcm9qZWN0Lm9yZy92MC5qcyI+PC9zY3JpcHQ+CiAgPHNjcmlwdCBjdXN0b20tZWxlbWVudD0iYW1wLWFkIiBzcmM9Imh0dHBzOi8vY2RuLmFtcHByb2plY3Qub3JnL3YwL2FtcC1hZC0wLjEuanMiIGFzeW5jPjwvc2NyaXB0PgogIDxsaW5rIHJlbD0iY2Fub25pY2FsIiBocmVmPSJodHRwczovL3ByZXZpZXcuYW1wLmRldi9kb2N1bWVudGF0aW9uL2NvbXBvbmVudHMvYW1wLWFkLmV4YW1wbGUuMS5odG1sIj4KICA8bWV0YSBuYW1lPSJ2aWV3cG9ydCIgY29udGVudD0id2lkdGg9ZGV2aWNlLXdpZHRoIj4KICA8c3R5bGUgYW1wLWJvaWxlcnBsYXRlPmJvZHl7LXdlYmtpdC1hbmltYXRpb246LWFtcC1zdGFydCA4cyBzdGVwcygxLGVuZCkgMHMgMSBub3JtYWwgYm90aDstbW96LWFuaW1hdGlvbjotYW1wLXN0YXJ0IDhzIHN0ZXBzKDEsZW5kKSAwcyAxIG5vcm1hbCBib3RoOy1tcy1hbmltYXRpb246LWFtcC1zdGFydCA4cyBzdGVwcygxLGVuZCkgMHMgMSBub3JtYWwgYm90aDthbmltYXRpb246LWFtcC1zdGFydCA4cyBzdGVwcygxLGVuZCkgMHMgMSBub3JtYWwgYm90aH1ALXdlYmtpdC1rZXlmcmFtZXMgLWFtcC1zdGFydHtmcm9te3Zpc2liaWxpdHk6aGlkZGVufXRve3Zpc2liaWxpdHk6dmlzaWJsZX19QC1tb3ota2V5ZnJhbWVzIC1hbXAtc3RhcnR7ZnJvbXt2aXNpYmlsaXR5OmhpZGRlbn10b3t2aXNpYmlsaXR5OnZpc2libGV9fUAtbXMta2V5ZnJhbWVzIC1hbXAtc3RhcnR7ZnJvbXt2aXNpYmlsaXR5OmhpZGRlbn10b3t2aXNpYmlsaXR5OnZpc2libGV9fUAtby1rZXlmcmFtZXMgLWFtcC1zdGFydHtmcm9te3Zpc2liaWxpdHk6aGlkZGVufXRve3Zpc2liaWxpdHk6dmlzaWJsZX19QGtleWZyYW1lcyAtYW1wLXN0YXJ0e2Zyb217dmlzaWJpbGl0eTpoaWRkZW59dG97dmlzaWJpbGl0eTp2aXNpYmxlfX08L3N0eWxlPjxub3NjcmlwdD48c3R5bGUgYW1wLWJvaWxlcnBsYXRlPmJvZHl7LXdlYmtpdC1hbmltYXRpb246bm9uZTstbW96LWFuaW1hdGlvbjpub25lOy1tcy1hbmltYXRpb246bm9uZTthbmltYXRpb246bm9uZX08L3N0eWxlPjwvbm9zY3JpcHQ+CjwvaGVhZD4KPGJvZHk+Cgo8YW1wLWFkCiAgd2lkdGg9IjMwMCIKICBoZWlnaHQ9IjI1MCIKICB0eXBlPSJpbmR1c3RyeWJyYWlucyIKICBkYXRhLXdpZHRoPSIzMDAiCiAgZGF0YS1oZWlnaHQ9IjI1MCIKICBkYXRhLWNpZD0iMTk2MjYtMzc5ODkzNjM5NCIKPgo8L2FtcC1hZD4KPGFtcC1hZAogIHdpZHRoPSIzMDAiCiAgaGVpZ2h0PSIyNTAiCiAgdHlwZT0iaW5kdXN0cnlicmFpbnMiCiAgZGF0YS13aWR0aD0iMzAwIgogIGRhdGEtaGVpZ2h0PSIyNTAiCiAgZGF0YS1jaWQ9IjE5NjI2LTM3OTg5MzYzOTQiCj4KPC9hbXAtYWQ+CiAgCjwvYm9keT4KPC9odG1sPg==) and devtools to check the value from the console using frames contexts.

Example page html
```html

amp-ad - Example 1




body{-webkit-animation:-amp-start 8s steps(1,end) 0s 1 normal both;-moz-animation:-amp-start 8s steps(1,end) 0s 1 normal both;-ms-animation:-amp-start 8s steps(1,end) 0s 1 normal both;animation:-amp-start 8s steps(1,end) 0s 1 normal both}@-webkit-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-moz-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-ms-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-o-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}body{-webkit-animation:none;-moz-animation:none;-ms-animation:none;animation:none}

```

### Relevant Logs

```shell

```

### Browser(s) Affected

_No response_

### OS(s) Affected

_No response_

### Device(s) Affected

_No response_

### AMP Version Affected

_No response_