The test is sometimes failing with this error:

OfflineStoreKitIntegrationTests.swift:262: Entitlement is not active: <EntitlementInfo: "
identifier=premium,
isActive=false,
willRenew=true,
periodType=PeriodType(rawValue: 1),
latestPurchaseDate=Optional(2025-07-17 15:45:36 +0000),
originalPurchaseDate=Optional(2025-07-17 15:45:36 +0000),
expirationDate=Optional(2025-07-17 15:45:36 +0000),
...
>

The error shows that the entitlement is not active while it should be active. But if you notice the dates in the log, latestPurchaseDate is exactly the same as expirationDate. This makes me think that, if the test runs fast enough, the lines

try await self.purchaseMonthlyProduct(allowOfflineEntitlements: true)
try self.testSession.forceRenewalOfSubscription(
    productIdentifier: await self.monthlyPackage.storeProduct.productIdentifier
)

can produce two transactions with the same purchase_date (and the first one having also the same expires_date as purchase_date). This has just happened to me when running the test locally:

DEBUG: ℹ️ PostReceiptDataOperation: Posting receipt (source: 'queue') (note: the contents might not be up-to-date, but it will be refreshed with Apple's servers):
{...,
"in_app_purchases":[

{"transaction_id":"7","is_in_intro_offer_period":false,"original_transaction_id":"6","purchase_date":"2025-07-17T17:09:33Z","original_purchase_date":"2025-07-17T17:09:33Z","expires_date":"2025-07-17T17:10:03Z","product_id":"com.revenuecat.monthly_4.99.1_week_intro","product_type":-1,"quantity":1},

{"transaction_id":"6","is_in_intro_offer_period":true,"purchase_date":"2025-07-17T17:09:33Z","expires_date":"2025-07-17T17:09:33Z","product_id":"com.revenuecat.monthly_4.99.1_week_intro","product_type":-1,"quantity":1}

],
...}

I suspect that, because both transactions have the same purchase_date, the first one may be randomly picked between the two. In this case, the test then would fail because that transaction (with same expires_date as purchase_date) doesn't grant the entitlement anymore.
This is confirmed by the test error message having periodType=PeriodType(rawValue: 1), which is the PeriodType.intro enum case and it showing that latestPurchaseDate == expirationDate. Only the first transaction is the intro offer one.

My reasoning is that by adding 1 second delay between the two transactions will make sure that the two transactions always have a different purchase_date, so that the second transaction is always the one being picked as the latest one