[Google Pixel 7 Pro] WoKoschekk:
Let me explain the error in v34.0.0:

As we all know the partition /super contains a few dynamic partitions. These dynamic partitions only can be handled in userspace by fastbootd mode (which is an executable binary inside the recovery's ramdisk). To flash /super there are two existing methods:

1. In bootloader mode by flashing a super.img to /super.

2. In fastbootd mode by flashing the single dynamic partitions inside of /super.

The firmware of a Pixel has got no super.img inside it, but instead of this another zip archiv (imagesXXX.zip) with all dynamic partitions and a super_empty.img. The super_empty.img is only a partition table of /super with all start/end blocks of the dynamic partitions. These start/end blocks may change with every new firmware build and that's why they are called "dynamic".

For flashing the Pixel's firmware the command fastboot flashall is used by Google. The flashing process starts in bootloader mode to flash bootloader.img + radio.img and then the flashing routine behind fastboot flashall has to decide whether to stay in bootloader mode or to switch into fastbootd mode. For Pixel devices fastboot should switch into fastbootd mode to flash the single dynamic partitions. But it won't and that's the error in v34.0.0.!

Instead of this fastboot remains in bootloader mode and tries flashing /super with a super.img. Let's have a look again at the error message that users get:

Sending sparse 'super' 1/1 (4194303 KB)
FAILED (Sparse file is too large or invalid)
fastboot: error: Command failed

Due to the fact that there's no super.img (neither as a raw image nor as a sparse image) fastboot outputs that error message above. The reason for getting this error you'll find in the source (line 249):

RetCode FastBootDriver::Download(const std::string& partition, struct sparse_file* s, uint32_t size,
size_t current, size_t total, bool use_crc, std::string* response,
std::vector<std::string>* info) {
prolog_(StringPrintf("Sending sparse '%s' %zu/%zu (%u KB)", partition.c_str(), current, total,
size / 1024));
auto result = Download(s, use_crc, response, info);
epilog_(result);
return result;
}
RetCode FastBootDriver::Download(sparse_file* s, bool use_crc, std::string* response,
std::vector<std::string>* info) {
error_ = "";
int64_t size = sparse_file_len(s, true, use_crc);
if (size <= 0 || size > MAX_DOWNLOAD_SIZE) {
error_ = "Sparse file is too large or invalid";
return BAD_ARG;
}

In short: If sparse_file size is 0 Byte or less (= no sparse_file) than output error text "Sparse file is too large or invalid".

Why does this error occure since v34.0.0?

The developer David Anderson thought it was a good idea not to boot into fastbootd mode as much as really, really needed. He wanted to save time during the flashing process (round about 50 sec.). See fully detailed in the spoiler below.

Spoiler content hidden.

But something went wrong during the execution of his code. As a result of this the flashing process ends with an error and without flashing the contents of /super.

Following the log of the repo /android/platform/system/core/refs/heads/master/./fastboot several changes were made till today. But we'll have to wait for the next update, which will (hopefully) be officially released soon.

I hope it is somewhat understandable. :-)