I can confirm this issue is still occurring for us on iOS 14.2 and is exactly as you described. Our share extension does not show up in the share sheet the first time you tap the share button. But if you dismiss the sheet and tap the share button again, then it will appear the second time. Sounds like it might be a iOS or Safari bug. Interestingly enough a very similar bug was introduced in macOS Catalina 10.15 which was ultimately fixed in 10.15.4. With that bug, share extensions that used the NSExtensionJavaScriptPreprocessingFile key simply didn't appear at all.

This appears to be randomly happening again. Curious if anyone else has been experiencing this problem lately. Found the following related logs in the Console: default    15:39:49.352585-1000    storeuid    <StoreKitClient: 0x7f8898d2d820>: Using active account. Sandbox: 1, receipt exists: NO, receipt is stub: NO default    15:39:49.354734-1000    commerce    storeuid[552] starting request app-receipt-create default    15:39:49.455240-1000    commerce    <CKStoreRequest: 0x7f94a1f64e10> https://p100-sandbox.itunes.apple.com/WebObjects/MZFinance.woa/wa/createAppReceipt default    15:39:49.877166-1000    commerce    StoreSession: StatusCode: 200; <private>; Environment: SB-MR; URL: https://p100-sandbox.itunes.apple.com/WebObjects/MZFinance.woa/wa/createAppReceipt error    15:39:49.879812-1000    storeuid    <ReceiptRefreshRequest: 0x7f889a1616e0>: No receipt data in response error    15:39:49.879883-1000    storeuid    <ReceiptRefreshRequest: 0x7f889a1616e0>: Error fetching receipt for X - Error Domain=com.apple.commerce.client Code=500 "(null)" default    15:39:49.880675-1000    storelegacy    StoreLegacy: Failed to renew receipt for application at path X : 'The operation couldn’t be completed. (com.apple.commerce.client error 500.)' error    15:39:49.930396-1000    diagnostics_agent    Invalid receipt [0 bytes]