Problem with BLE on Argon

Problem with BLE on Argon
Hello, everyone!
When sending data using BLE, the data does not arrive completely - part of the data is lost.
By UART, a string is received into the buffer, the buffer size is 700-800 characters. After that, the UART buffer is sent using BLE.
I accept data from BLE using the Adafruit Bluefruit app on my smartphone - I receive average up to 100 characters - the rest of the characters is being lost.
I use an example from Particle Argon (BLE Log Handler) adapted to oneself. Link: https://docs.particle.io/tutorials/device-os/bluetooth-le/
I’m transmitting the string via BLE using function Log.printf().
Maybe someone has an example of how to receive a large stream of data via UART and send using BLE?
Could you please explain, what am I doing wrong?