TPS61291: About Boost mode on high Vin condition

Please let me know about two points below for TPS61291 operation;

【Condition】

・Vout： 3.3V

・EN/BYP pin：Held High level input (Boost mode)

・When Vin is 3.3V～3.78V, Vout occur large ripple. (Vout waveform is attached below)

・When Vin is more than 3.8V, Vout ripple was disappeared.

【Question】

①What TPS61291 operation in the condition of Vin≧Vout and Boost mode setting?

　I want to know the reason of large ripple on  Vin: 3.3V～3.78V. 

②When Vin>3.8V, I think that TPS61291 changed Bypass mode, is it correct?

　If correct, is there information  for these operation?

Best regards,

Satoshi