I want to know why a transaction with a large number of spans is sent by the SDK but does not show up in Sentry (or shows incomplete span data), and whether there is a span limit.

Yes, transactions have a 1000 span limit, which is configurable. This limit exists for avoiding transactions that never finish (in platforms that keep a transaction open for as long as spans are open), preventing OOM errors, and generally avoiding degraded application performance. This, and other SDK limitations, can be found in our developer documentation.

To work around this, please check your SDK's Basic Options docs to see if it has an option to change the maximum amount of allowed spans. Otherwise, you will need to use your SDK's version of before send transaction to set up custom logic that would delete any excess spans.